With a recent tendency to design various electric equipments into micro-electronic forms, a battery has been housed in an electric equipment and integrated with the electric equipment and its circuit, as represented by power sources for memory back-up of various electric equipments. For this reason, a demand for minimizing a size, a weight and a thickness of battery and a request for battery having a large energy density have been increasing. In a field of primary battery, a small-sized and light-weight battery such as a lithium battery has already been put to practical use, however, its application field is limited to a small region. Under these circumstances, in a field of secondary battery, a battery utilizing nonaqueous electrolyte, which can be made smaller in size and weight, attracts public attention at present as an alternate battery in place of a conventional lead-battery and a nickel-cadmium battery. However, in the battery utilizing the nonaqueous electrolyte, an electrode active material which can satisfy practical physical properties such as a cycle characteristic and a self-discharge characteristic, has not been found yet. Therefore, investigations are carried on still now in many research organizations.
Here, in order to obtain a small-sized and light-weight battery having a large energy density and a high reliability, it is necessary to examine the following problems (1) and (2).
(1) Problem of electrode active material and electrode
(2) Problem of electrolyte
As for the problem (1), the inventor examined a film type battery, that is, a battery having unit cells with thicknesses of 100 to 500 microns and called also as "sheet-shaped" battery. In this kind of battery, however, such problems arose that a manufacture of metallic lithium foil having a desirable performance was somewhat difficult from a technical point of view and that a manufacturing process of battery became complicated. Further, in the secondary battery, such a problem arose that a formation of lithium dendrite and a passivation of interface took place so that use of metallic lithium was restricted. Therefore, investigations on alloys including lithium metals as represented by lithium-aluminum, lithium-zinc and lithium-tin, are being carried on actively. However, the electrode was cracked or broken into fine pieces due to repeated charging and discharging so that the cycle characteristic was not improved even when these alloys were used, because these alloys have small strengths as represented by the lithium-aluminum alloy. As an alternate method for restricting the formation of lithium dendrite, investigations on selection of electrolyte salt and improvement in separator are being tried. As for the separator among them, it is attempted now to restrict the formation of lithium dendrite by laminating non-woven fabrics made of polypropylene and non-woven fabrics made of glass fiber, which have so far been used. However, a substantial solution has not been found yet.
Accordingly, electrode active materials utilizing intercalation or doping phenomenon of layer compound are specially studied now in many research organizations. These materials are expected for their extremely excellent charge/discharge cycle characteristics, because a theoretically complicated chemical reactions does not occur at time of electro-chemical reaction in the charging and discharging. Use of carbon material as the electrode active material is a method turned up, during the studies as mentioned above, as a solution for problems of cycle characteristic and self-discharge characteristic of the electrode active material. Features of this carbon material are a high doping capacity, a low self-discharge rate and an excellent cycle characteristic. A feature to be specially mentioned is that it has a base-potential extremely near to that of metallic lithium.
On the other hand, the problem (2) is as described below. A liquid electrolyte, especially prepared by dissolving ionic compound in an organic electrolyte, has so far been used for an electrolyte for a battery utilizing electro-chemical reaction and electro-chemical devices other than the battery, such as electric double-layer capacitor and electro-chromic element etc. However, since there have been troubles such as leakage of electrolyte to battery outside and easiness of elusion and evaporation of electrode material etc. when the liquid electrolyte has been used, problems of long-term reliability and flying-around of electrolyte in a sealing process have remained unsolved. As a means to solve these problems, that is, a means to improve a solution-leakage resistance and a long-term reliability, an ion-conductive high-molecular compound having a large ionic conductivity has been reported and further studied.
Ion-conductive high-molecular compounds being studied now are straight-chain polymer, network crosslink polymer or comb-shaped polymer, of homopolymer or copolymer having ethylene-oxide as its basic unit. It is proposed and practiced that crystallization is avoided by making the compound into forms of network crosslink polymer or comb-shaped polymer for the purpose of increasing the ionic conductivity at a low temperature. Especially, the ion-conductive high-molecular compound using the network crosslink polymer has a large mechanical strength and is excellent in the ionic conductivity at a low temperature, so that it is useful.
Electro-chemical cells using the ion-conductive high-molecular compound are described widely in many patent documents. There are, for example, U.S. Pat. No. 4,303,748 (1981) by Armand etc., U.S. Pat. No. 4,589,197 (1986) by North, and U.S. Pat. No. 4,547,440 (1985) by Hooper etc. A feature which can be mentioned for these cells is the use of ion-conductive high-molecular compound prepared by dissolving ionic compound into high-molecular compound having a polyether structure.
In order to use the ion-conductive high-molecular compound as the electrolyte for batteries utilizing the electro-chemical reaction and the electro-chemical devices other than the battery, it is required for the high-molecular compound to have both the high ionic conductivity and the high mechanical property (mechanical strength and flexibility etc.). However, these properties contradict to each other. In many patent documents described above, for example, the compound is operated principally in a state of high temperature because an ionic conductivity at a temperature lower than room temperature decreases down below a practical range. Therefore, as a simple method for improving the ionic conductivity for example, a method is proposed, in Published Patent Application (KOKAI) No. 59-149601, Published Patent Application (KOKAI) No. 58-75779, U.S. Pat. No. 4,792,504 etc., that an organic solvent (specially preferably, an organic solvent with high permittivity) is added to the ion-conductive high-molecular compound to keep a solid state. In this method, however, the ionic conductivity is improved positively, to be sure, but the mechanical strength is worsened extremely. While, in the electrode active material utilizing intercalation or doping phenomenon of the layer compound, expansion and contraction of the electrode active material are produced accompanied by charging and discharging. To cope with this problem, it is required to improve mechanical strengths of the electrode and the electrolyte.
When the ion-conductive high-molecular compound is used as the electrolyte for electro-chemical devices, it becomes necessary to make the electrolyte into a film shape in order to reduce an internal resistance. Especially, this is important for the film type battery. In case of the ion-conductive high-molecular compound, it is possible to work its uniform film easily into a voluntary shape, and various methods for this purpose are known. There are several methods, for example, such as a method in which a solution of the ion-conductive high-molecular compound is cast and its solvent is evaporated and removed, a method in which polymeric monomer or macromer is applied on a substrate to be heated and polymerized, or a method in which curing is done by means of irradiation of activated ray. It is possible to obtain an uniform film when these methods are used. However, a fine short-circuiting has sometimes occurred due to breakage of the electrolyte layer caused by its compression deformation when practically laminating the ion-conductive high-molecular compounds in between the electrodes to assemble the battery and electro-chromic element etc. Accordingly, in order to make the ion-conductive high-molecular compound into an uniform film, the improvement of mechanical strength is important in addition to the ionic conductivity.
This invention is made in consideration of the above present circumstances, and an object of it is to provide a secondary battery excellent in charge/discharge characteristic and long-term reliability and with high performance.